Ya know, I've been working with Pro Tools 6.x under OSX for quite a while, and go back to 5.3.1 under OS 9.2.2 (I have a dual boot G4) when I need to record using a video card that only has drivers under OS9.
Anyway, my point is that I'm always shocked at how much faster, more fluid, and easier on the eyes version 5.3.1 is than the OSX version. I'd say stick with OS9 as long as you can.